Optical transmitter

ABSTRACT

An optical transmitter that can perform high-speed ON/OFF control of bias light that is input to an optical modulator. A high-speed current switching circuit performs a high-speed ON/OFF control of a drive current of an LD according to an LD ON/OFF signal. Thus, high-speed ON/OFF control of an optical output from the LD can be performed. A temperature detecting current generation circuit detects the ambient temperature, and generates the drive current adjusted according to the ambient temperature detected thereby so that the optical output from the LD is not affected by the ambient temperature.

TECHNICAL FIELD

The present invention relates to an optical transmitter employing anoptical modulator, and more particularly, to an optical transmitter inwhich a high-speed ON/OFF control of the emission of bias light that isto be modulated is performed.

BACKGROUND ART

Optical transmitters are provided in, for example, an opticalcommunication system, and converts electrical signals to optical signalsso as to transmit the thus converted optical signals. Conventionaloptical transmitters are described in, for example, Patent Documents 1and 2.

Patent Document 1 describes an optical transmitter that includes a laserdiode (LD) as an optical source; a transistor into which an ON/OFFsignal of the LD (hereinafter, “LD ON/OFF signal”) is input and thatcontrols a drive current of the LD accordingly; and an externalmodulator that modulates light output from the LD with an input signal.The circuit of such an optical transmitter is configured in such amanner that the collector of the transistor is connected to the cathodeof the LD, a load resistor is provided between the emitter of thetransistor and the ground (GND), and an LD ON/OFF signal is input to thebase of the transistor. Thus, ON/OFF of the laser diode (LD) iscontrolled.

Patent Document 2 describes an optical transmitter that further includesa photodiode (PD) that detects an optical output from the LD; and afeedback control circuit such as a feedback automatic power control(APC) circuit connected to the PD, in addition to the configuration ofthe optical transmitter described in Patent Document 1. In such anoptical transmitter, an LD drive current is controlled by using thefeedback control circuit according to the detection results of theoptical output from the LD detected by the PD, to thereby control theoptical output from the LD to be made constant. In this opticaltransmitter also, the optical output can be turned ON or OFF accordingto the LD ON/OFF signal.

[Patent Document 1] Japanese Patent Application Laid-open No. H9-83050

[Patent Document 2] Japanese Patent Application Laid-open No. 2000-89178

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

The conventional technologies described above, however, have a followingproblem. That is, in the conventional optical transmitters, it isdifficult to perform high-speed ON/OFF control of bias light that isinput to the external modulator.

For example, in the optical transmitter disclosed in Patent Document 1,the LD ON/OFF function thereof is not meant for high-speed operation.Therefore, ON/OFF stabilization of a drive current takes time, and thus,high-speed ON/OFF control cannot be performed therein. Actually, when awaveform of the LD drive current for the LD ON/OFF signal is examined,it can be determined that it takes time for a current value with thesignal being ON to become stable at a predetermined value when thesignal is switched from OFF to ON, and it takes time for a currentthereof to become 0 when the signal is switched from ON to OFF.

In the optical transmitter disclosed in Patent Document 2, since afeedback APC is used to stabilize the optical output from the LD, a loopcircuit is disconnected according to the LD ON/OFF signal. Therefore, ittakes some time for the drive current to return to a stable state. As aresult, the ON/OFF stabilization of the drive current still requiressome time.

In view of the foregoing, an object of the present invention is toprovide an optical transmitter in which high-speed ON/OFF control ofbias light that is input to the optical modulator can be performed.

Means for Solving Problem

In order to solve the aforementioned problems, an optical transmitteraccording to the present invention is constructed in such a manner thatit comprises: a light-emitting device; an optical modulator thatmodulates light output from the light-emitting device by using an inputsignal; a drive current switching controller that performs ON/OFFswitching control of a drive current of the light-emitting device, byusing as an input an ON/OFF signal that controls ON/OFF of an opticaloutput of the light-emitting device, in response to switching of theON/OFF signal; and a drive current adjusting and generating unit thatdetects ambient temperature, and generates the drive current that isadjusted according to the ambient temperature detected thereby.

Effect of the Invention

According to the present invention, a drive current is generated by adrive current adjusting and generating unit, and an ON/OFF switchingcontrol of the drive current of a light-emitting device is performed bya drive current switching and generating unit according to an ON/OFFsignal. Therefore, high-speed ON/OFF control of the optical output fromthe light-emitting device can be performed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram of an optical transmitter according to a firstembodiment of the present invention.

FIG. 2 is a block diagram of an optical transmitter according to asecond embodiment of the present invention.

FIG. 3 is a circuit diagram of a configuration of a high-speed currentswitching circuit according to the first embodiment.

FIG. 4 is a schematic of a simulation result of an LD drive currentwaveform for an LD ON/OFF signal in the circuit configuration shown inFIG. 3.

FIG. 5 is a block diagram of a conventional optical transmitterdisclosed in Patent Document 1.

FIG. 6 is a schematic of a simulation result of an LD drive currentwaveform for an LD ON/OFF signal in a circuit configuration shown inFIG. 5.

FIG. 7 is a block diagram of a conventional optical transmitterdisclosed in Patent Document 2.

EXPLANATIONS OF LETTERS OR NUMERALS

-   1, 100 external modulator-   2, 102 LD-   3 high-speed current switching circuit-   4 temperature detecting current generation circuit-   5 signal head detecting circuit-   6, 7, 9, 11, 113 transistor-   8, 10, 12, 114 load resistor-   20 differential circuit-   21 current mirror-   116 PD-   117 feedback control circuit

BEST MODE(S) FOR CARRYING OUT THE INVENTION

An optical transmitter according to the present invention is describedbelow in detail with reference to the accompanying drawings. The presentinvention is, however, not limited thereto.

First Embodiment

FIG. 1 is a block diagram of an optical transmitter according to a firstembodiment of the present invention. FIG. 3 is a circuit diagram of aconfiguration of a high-speed current switching circuit according to thefirst embodiment.

As shown in FIG. 1, the present embodiment includes an externalmodulator 1; a device such as an LD 2 that serves as a light-emittingdevice that converts a current signal into light; a high-speed currentswitching circuit 3 that serves as a drive current switching controller;and a temperature detecting current generation circuit 4 that serves asa drive current adjusting and generating unit.

The LD 2 is an optical source, and an optical output from the LD 2 isinput to the external modulator 1. The external modulator 1 is anoptical modulator that modulates the light output from the LD 2 by usingan input signal that is a transmission data.

The high-speed current switching circuit 3 is connected to the cathodeof the LD 2, and performs high-speed ON/OFF control of a drive currentof the LD 2 by using an LD ON/OFF signal as an input signal, which isused to perform ON/OFF control of an optical output from the LD 2. Thehigh-speed current switching circuit 3 is connected to the temperaturedetecting current generation circuit 4. The temperature detectingcurrent generation circuit 4 detects an ambient temperature, and sets adrive current at the time when the LD 2 is turned ON so that an opticaloutput level of the LD 2 is not affected by a fluctuation of the ambienttemperature. In other words, the temperature detecting currentgeneration circuit 4 generates a current according to the detectionresults of the ambient temperature, and the high-speed current switchingcircuit 3 sets a drive current at the time when the LD 2 is turned ON,according to the current generated by the temperature detecting currentgeneration circuit 4.

The operation performed by the configuration shown in FIG. 1 isdescribed below. DC emitted light generated by the LD 2 is input to theexternal modulator 1, and then modulated by an input signal that isinput to the external modulator 1 through an input signal terminal, andthe thus modulated optical output is further transmitted therefrom. Thehigh-speed current switching circuit 3 performs a high-speed ON/OFFcontrol of a drive current of the LD 2 (LD drive output) according to anLD ON/OFF signal (LD ON/OFF signal input) that is input thereto, so asto perform thereby a high-speed ON/OFF control of the optical outputfrom the LD 2. The temperature detecting current generation circuit 4sets an optimal preset value for each temperature level so that anoptical output level of the LD 2 is constant, not being dependent ontemperature. The high-speed current switching circuit 3 uses an optimalcurrent that is set by the temperature detecting current generationcircuit 4 as an input (output current setting input), and accordinglysets an LD drive output that is a drive current of the LD 2. Thus, anoptical output level of the LD 2 at the time when the LD 2 is turned ONis controlled so as to be made constant without being dependent ontemperature.

FIG. 3 is a circuit diagram of a configuration of the high-speed currentswitching circuit according to the present embodiment. As shown in FIG.3, the high-speed current switching circuit 3 is equipped with adifferential circuit 20 and a current mirror circuit 21.

The differential circuit 20 includes a first transistor 6, a secondtransistor 7, and a load resistor 8. The first and the secondtransistors 6 and 7 have a common emitter, and an end of the loadresistor 8 is connected to the collector of the transistor 7. Thecollector of the transistor 6 is connected to an LD drive outputterminal and to the cathode of the LD 2. The potential of the other endof the load resistor 8 is set to be identical to the potential on theanode side of the LD 2. An LD ON/OFF signal input terminal includes 2terminals. One terminal is connected to the base of the transistor 6,and the other end is connected to the base of the transistor 7. Theemitter that is common to the transistors 6 and 7 is connected to thecurrent mirror circuit 21.

The current mirror circuit 21 includes a first transistor 9; a secondtransistor 11; a load resistor 10 that is provided between thetransistor 9 and the GND; and a load resistor 12 that is providedbetween the transistor 11 and the GND. The collector of the transistor 9is connected to the emitter that is common to the transistors 6 and 7,and the emitter of the transistor 9 is connected to the GND through theload resistor 10. The collector of the transistor 11 is connected to anoutput current setting input terminal and further to the temperaturedetecting current generation circuit 4. The emitter of the transistor 11is connected to the GND through the load resistor 12, and the base andthe collector of the transistor 11 are connected to each other. Thebases of the transistors 9 and 11 are connected to each other.

The operation performed by the high-speed current switching circuit 3 isdescribed below. A current generated by the temperature detectingcurrent generation circuit 4 is input to the current mirror circuit 21through the output current setting input terminal. In the current mirrorcircuit 21, a current that is identical to the current flowing in thetransistor 11 flows in the transistor 9 in the same direction.Therefore, the current that is input to the output current setting inputterminal is supplied to the differential circuit 20 through the currentmirror circuit 21. The differential circuit 20 switches current pathsaccording to the LD ON/OFF signal that is input through the LD ON/OFFsignal input terminal, and outputs the current that is suppliedthereinto. That is, for example, when an ON signal is input from the LDON/OFF signal input terminal, a positive phase and a negative phasesignals are applied to the bases of the transistors 6 and 7,respectively, and thus, the drive current supplied from the currentmirror circuit 21 flows into the transistor 6, but not into thetransistor 7. Therefore, when the LD 2 is turned ON, the LD drivecurrent that is the LD drive output is determined according to thecurrent that is input to the output current setting input terminal. Whenan OFF signal is input thereto through the LD ON/OFF signal inputterminal, the signals that are input to the bases of the transistors 6and 7 oppose to the case described above, and the LD drive current isnot output to the LD. Thus, in the present embodiment, high-speed ON/OFFcontrol of the LD drive current is performed by employing the high-speedcurrent switching circuit 3 including, for example, the differentialcircuit 20.

FIG. 4 is a schematic of a simulation result of an LD drive currentwaveform for the LD ON/OFF signal, explaining an effect of the circuitshown in FIG. 3. In the graph of the LD ON/OFF signal in the upper halfof FIG. 4, the horizontal axis denotes time (ns) and the vertical axisdenotes voltage (V). Note that ns denotes nanosecond. The LD ON/OFFsignal is 1.4 (V) at the OFF state during 0 to 1 (ns), 1.8 (V) at the ONstate during 1 to 3 (ns), and 1.4 (V) at the OFF state during 3 to 5(ns). In the graph of the LD drive current waveform in the lower half ofFIG. 4, the horizontal axis denotes time (ns) and the vertical axisdenotes current (mA). It can be observed that when the LD ON/OFF signalis switched from the OFF state to the ON state, the drive current isstabilized only 100 picoseconds after the start of the ON signal, sothat a desired drive current can be started to flow. In fact, it can beunderstood that the drive current is already stabilized at the point ofMO (1.1 nanoseconds, 79.8 milliamperes), and that high-speedstabilization of the drive current can be performed. On the other hand,when the LD ON/OFF signal is switched from the ON state to the OFFstate, the current value already reads 14.6 microampere in only 100picoseconds after the LD ON/OFF signal is switched to the OFF signal(see the point M2 (3.1 nanoseconds, 14.6 microamperes)). As describedlater, it can be understood that the current value is made less than ahalf of the conventional value. Thus, higher-speed switching of the LDdrive current can be performed compared with a conventional opticaltransmitter.

To make the difference between effects of the present embodiment andeffects of conventional optical transmitters clear, the opticaltransmitters disclosed in Patent Documents 1 and 2 are described withreference to FIGS. 5 to 7.

FIG. 5 is a block diagram of the conventional optical transmitterdisclosed in Patent Document 1. As shown in FIG. 5, the conventionaloptical transmitter includes an LD 102 that serves as an optical source;an external modulator 100 that modulates light output from the LD 102 byusing an input signal, and outputs the light thus modulated; atransistor 113 into which an ON/OFF signal of the LD 102 is input andthat controls a drive current of the LD 102; and a resistor providedbetween the transistor 113 and the GND.

The collector of the transistor 113 is connected to the cathode of theLD 102, and the resistor 114 is provided between the emitter of thetransistor 113 and the GND. An LD ON/OFF signal is input to the base ofthe transistor 113, and the ON/OFF control of the LD 102 is performedaccording to the LD ON/OFF signal.

FIG. 6 is a schematic of a simulation result of an LD drive currentwaveform for an LD ON/OFF signal in a circuit configuration shown inFIG. 5. The LD ON/OFF function in a conventional optical transmitter,however, is not meant for a high-speed operation. Thus, in FIG. 6, theresult of the simulation is shown in which an ultra-high speedtransistor having a transition frequency of ft=100 gigahertz is used inthe circuit configuration shown in FIG. 5.

In the graph of the LD ON/OFF signal in the upper half of FIG. 6, thehorizontal axis denotes time (ns), and the vertical axis denotes voltage(V). The LD ON/OFF signal is 0 (V) at the OFF state during 0 to 1 (ns),at a stable voltage of 1.5 (volts) at the ON state during 1 to 3 (ns),and 0 (volt) at the OFF state during 3 to 5 (ns). In the graph of the LDdrive current waveform in the lower half of FIG. 6, the horizontal axisdenotes time (ns), and the vertical axis denotes current (mA). In thisconventional configuration, when the LD ON/OFF signal is switched fromthe OFF state to the ON state, it takes more than about 1 (ns) until acurrent value of the LD drive current waveform becomes generally stableto be a constant value. In other words, a drive current is notstabilized from the time at which the signal is switched into the ONstate to the time M1 (2 nanoseconds, 73.8 milliampere). It can beunderstood that it takes considerably more time in comparison with thepresent embodiment (the current value is stabilized about in 100 (ps)).When the LD ON/OFF signal is switched from the ON state to the OFFstate, a current of about 24 (μA) is flowing at 0.1 (ns) after the LDON/OFF signal is switched into the OFF state, as shown in the point M3(3.1 nanoseconds, 24.2 microampere). Thus, it can be understood that thestabilization takes considerably more time in comparison with thepresent embodiment (14.6 μA) at 0.1 (ns) after the signal is switchedinto the OFF state). At the point M1 (250.4 picoseconds, 5.801picoamperes), a current value is about 5.9 (ps), that is, in a stablestate at the time when the LD ON/OFF signal is switched into the OFFstate. Thus, in the optical transmitter disclosed in Patent Document 1,current ON/OFF stabilization is time consuming, and thus, high-speedON/OFF control is not achieved. On the other hand, in the presentembodiment, it can be understood that high-speed ON/OFF control isachieved.

FIG. 7 is a block diagram of the conventional optical transmitterdisclosed in Patent Document 2. As shown in FIG. 7, the conventionaloptical transmitter is equipped with a photodiode (PD) 0116 that detectsan optical output of the LD 102; a feedback control circuit 117 such asa feedback APC circuit connected to the PD 116; and a transistor 115connected to the feedback control circuit 117, in addition to theconfiguration shown in FIG. 5.

The collector of the transistor 115 is connected to the cathode of theLD 102, and the emitter of the transistor 115 is connected to thecollector of the transistor 113. The base of the transistor 115 isconnected to the feedback control circuit 117. The other constituentelements are identical to those shown in FIG. 5. Therefore, the samereference numerals are used for the identical constituent elements, andtheir detailed descriptions are omitted.

In the conventional optical transmitter disclosed in Patent Document 2,that is configured as described above, the optical output emitted fromthe LD 102 is detected as photoelectromotive force generated in the PD116. Based on the detection results of the optical output, the LD driveoutput is controlled by using the feedback control circuit. Thus, theoptical output of the LD is controlled so as to be constant. In theconfiguration shown in FIG. 7, however, the feedback APC is used tostabilize the optical output, and the loop circuit is disconnected dueto the LD ON/OFF signal. Therefore, unfortunately, it takes time of themillisecond-order until the optical output is stabilized, and high-speedON/OFF control is thus not achieved.

As described above, in the present embodiment, it is made possible tocontrol high-speed ON/OFF of the drive current of the LD 2 by providingthe high-speed current switching circuit 3. As a result, high-speedON/OFF control of the optical output from the LD 2 can be performed. Thetemperature detecting current generation unit 4 is provided in theoptical transmitter, and the drive current of the LD 2 is adjusted inaccordance with the ambient temperature. Thus, the optical output fromthe LD 2 can be controlled so as to be constant without being dependenton the ambient temperature.

Second Embodiment

FIG. 2 is a block diagram of an optical transmitter according to asecond embodiment of the present invention. As shown in FIG. 2, thepresent embodiment further includes a signal head detecting circuit 5,in addition to the first embodiment shown in FIG. 1. The signal headdetecting circuit 5 is connected to the input signal terminal and to thehigh-speed current switching circuit 3. In FIG. 2, the same referencenumerals are used for the identical constituent elements shown inFIG. 1. The circuit configuration of the high-speed current switchingcircuit 3 according to the present embodiment is similar to that in thefirst embodiment, for example, the one shown in FIG. 3.

The operation performed in the present embodiment is described below. Aninput signal that is input to the external modulator 1 is also input tothe signal head detecting circuit 5. The signal head detecting circuit 5detects an identification signal that indicates the head position of theinput signal, and generates an LD ON/OFF signal synchronously with thedetection of the identification signal. Then, the signal head detectingcircuit 5 outputs the LD ON/OFF signal generated thereby to thehigh-speed current switching circuit 3.

In the present embodiment, the signal head detecting circuit 5 detectsan identification signal that indicates the head position of an inputsignal, and generates an LD ON/OFF signal. Therefore, an optical outputcan be easily obtained that is synchronized with the input signal. Theother configurations, the operations, the effects, and the like of thepresent embodiment are identical to those in the first embodiment.

INDUSTRIAL APPLICABILITY

As described above, an optical transmitter according to the presentinvention is useful as an optical transmitter that can performhigh-speed ON/OFF control of bias light that is input to an opticalmodulator, and can be preferably applied to an optical communicationsystem.

1. An optical transmitter comprising: a light-emitting device; anoptical modulator that modulates light output from the light-emittingdevice by using an input signal; a drive current switching controllerthat performs ON/OFF switching control of a drive current of thelight-emitting device, by using an ON/OFF signal as an input thatcontrols ON/OFF of an optical output of the light-emitting device, inresponse to the switching of the ON/OFF signal; and a drive currentadjusting and generating unit that detects ambient temperature, andgenerates a drive current that is adjusted according to the ambienttemperature detected thereby.
 2. The optical transmitter according toclaim 1, wherein the drive current switching controller includes acurrent mirror circuit and a differential circuit that is connected tothe current mirror circuit, the current mirror circuit supplies to thedifferential circuit a drive current that is generated by the drivecurrent adjusting and generating unit and input from the drive currentadjusting and generating unit, and the differential circuit controls thedrive current that is output from the current mirror circuit to thelight-emitting device, according to the ON/OFF signal.
 3. The opticaltransmitter according to claim 1, further comprising a signal headdetecting circuit that detects an identification signal that indicates ahead position of the input signal, generates the ON/OFF signalsynchronously with the detection of the identification signal, andoutputs the ON/OFF signal to the drive current switching controller. 4.The optical transmitter according to claim 2, further comprising asignal head detecting circuit that detects an identification signal thatindicates a head position of the input signal, generates the ON/OFFsignal synchronously with the detection of the identification signal,and outputs the ON/OFF signal to the drive current switching controller.